1. Field of the Invention
The present invention relates to an optical system. More specifically, the present invention relates to a photographic optical system (shooting optical system) of an image pickup apparatus, such as a silver-halide film camera, a digital still camera, a video camera, or a digital video camera.
2. Description of the Related Art
It is desired by the market that a photographic optical system used for a single-lens reflex digital camera or a single-lens reflex film camera has a wide angle of view and a long back focus. In this regard, as a photographic optical system having a wide angle of view and a long back focus, Japanese Patent Application Laid-Open No. 06-082689 and Japanese Patent Application Laid-Open No. 2002-287031 each discuss a retrofocus type photographic optical system.
A retrofocus type photographic optical system generally has an asymmetrical lens configuration. More specifically, such a retrofocus type photographic optical system includes a lens unit having a negative refractive power in front of an aperture stop (at a position in the photographic optical system closer to an object side) and a lens unit having a positive refractive power behind the aperture stop (at a position in the photographic optical system closer to an image side). The conventional photographic optical system discussed in each of Japanese Patent Application Laid-Open No. 06-082689 and Japanese Patent Application Laid-Open No. 2002-287031 implements a photographic optical system having a wide angle of view and a long back focus.
In a conventional photographic optical system, if an absolute value of the negative refractive power of the front lens unit becomes large, the amount of various aberrations occurring therein may become large. In particular, in a retrofocus type photographic optical system, the wider the angle of view becomes, the more asymmetrical the arrangement of the refractive powers may become. Therefore, in this case, a large amount of various aberrations, such as coma, astigmatism, and spherical aberration, may arise.
For example, in a conventional retrofocus type photographic optical system whose shooting angle of view is about 84° and whose F number is 1.4, a large amount of spherical aberration may occur in the front lens unit having a negative refractive power and the rear lens unit having a positive refractive power, which is located behind the aperture stop. Accordingly, in such a conventional photographic optical system, it is very difficult to effectively and balancedly correct various aberrations, such as spherical aberration.
In order to solve such a problem, it is useful to locate a lens having a strong positive refractive power in front of the aperture stop at a position distant therefrom. With this configuration, negative spherical aberrations occurring in the front lens unit having a negative refractive power and the rear lens unit having a positive refractive power located behind the aperture stop can be corrected.
However, if a lens having a strong positive refractive power is located distant from the aperture stop, the height of incidence of an off-axis light flux into the positive lens may become high. As a result, spherical aberration can be corrected. However, the amount of various aberrations, such as coma and astigmatism, occurring in the lens unit may be increased. Accordingly, in this case, it is difficult to obtain a high-quality image throughout the image plane.